Radioimmunotherapy treats cancerous tissues by injecting monoclonal antibodies labeled with charged particle emitting radionuclides. However, for the past 30 years, nuclear medicine has replied on crude anatomic models that originated in late 60's. Recent data show that doses calculated using these models are NOT correct for a very large portion of the patient population. On the other hand, CT, MR and cryosection images from the Visible Human Project (VHP) have provided realistic and most complete digital database on human anatomy. Therefore, it is hypothesized that whole-body patient models, constructed from extremely fine images of the Visible Human Project, can dramatically improve radiation dosimetry for patients undergoing internally administered radioisotope therapy. Although a previous study has solved critical issue on handling huge VHP images for radiation transport, little information exist on how to translate the recent advance into patient dosimetry for nuclear medicine. To address these needs, a research project is proposed to achieve the following Specific Aims: 1) To develop an adult male and an adult female patient whole-body computational model by processing, segmenting and classifying the 0.33 mm x 0.33 mm resolution cryosection images of the male and female cadavers obtained from the VHP. 2) To implement these 3-D, high-resolution, segmented tomographic patient models into the state-of-the-art radiation transport code packages to calculate selected nuclear medicine dosimetric parameters. 3) To evaluate the degrees to which improved anatomical representation of internal organ structures influences estimate of organ dose in adult patients who are undergoing a typical nuclear medicine procedure.